I was wondering what happen to soldiers when whole their regiment got wiped out. Let's say that ten soldiers will survive. What will happen to them? Will they get reasigned to another regiment? What if another regiment like theirs is half galaxy away?

A number of things could happen.

Retirement comes to mind, being allowed to become citizens on what ever planet they are on at the time, most likely being taken in by a noble house, who wants a few experienced gun thugs. Or being shoved into the PDF or just in general being allowed to just go about as normal citizens.

Being dissolved into anther Regiment or a mixed regiment- Anther likely thing would be they are put into a regiment, that is made up of the left over of possibly hundreds of regiments. A good book which mentions the subject is angel of fire, by William king. With out to many spoilers there's a few scenes towards the end of the first act as it were where this happens.

A Rogue trader May take them on for his or her own private army. But then if that happens, you might just want to play rogue trader, with a bit of cross over.

A inquisitor could take them as a disposable kill team. But then., might as well play dark heresy.

They could be tried and sent to a penal legion if they were found not doing there duty , as why would a single squad live ? maybe they fled the field .

If they have done extremely well and are highly decorated soldiers, the general of the over all command of imperial forces on the planet or area, may taken them on as guards.

and on the very slim chance if they are amazing soldiers with more medals that can fit on there uniform, they may be allowed to go back to there home world as hero's.

But If the Regiment in only war is gone, and you are left with just a hand full of guys, I would say ever call it a day, and start a new campaign with new characters and a regiment, or see if you can move into a new gaming system with in 40k, other wise you end up just playing other fantasy flight 40k games, just using only war.

No regiment matching them, say they're a light regiment. Slap a new cap badge and some specialized equipment on them and. 'Hey, we heard you line regiments are having difficulty with scouting duties. Here, some scouting specialists.' Retirement to the PDF or thuggery seems likely. A short story leaps to mind where a rich kid is sent to his ex-guard Grandad, who was a war hero, but hates what he had to do to get that accolade. He was a merc, and now just drinks and eats at a restaurant for free because he knows the owner and no-one is brave enough to tell him to stop. Mercing out your skills could well be a good option. Otherwise, see the Commissars post.

I believe that the proper thing to do would be to merge them into another pre-existing regiment. It doesn't matter overly much to the Departmento Munitorum that they're Line Infantry and the regiment they get merged into is Drop Troops. Fly or die, son.

That being said, see both previous posts for things that may actually happen, because what formally should happen or what the rules say is a far cry from what might happen. It also largely depends on how the situation came about. What killed the regiment? Where was it destroyed? Are there other regiments nearby? Who saved the survivors?

If the entire regiment is wiped out, I mean, the remaining guardsmen might not even know where to go or who to call in order to escape the planet. They might not even want to. If this happens to a penal regiment, I can see the legionnaires just chucking their uniforms. Or explode, if they have collars on.

Few options:

a) The regiment is disbanded: If Munitorium does not think that these guardsmen are worth the trouble they might just decide to take their regimental equipment back into armoury and retire them onto the world they are on. (Yay retirement and colonising the planet! Hopefully it was some place nice...) "Notify Cadia that 'Cadian 145th' can now be reused as the name of a regiment, the regiment's flag will be shipped back to Cadia."

b) Mix with another regiment(s): Usually Munitorium tries to match similar regiments. Two line regiments? "Together you go Krieg 162th and Catchan 54th. Hope you boys get along, both regiments are from a death world after all..." Occasionally they might mix and match specialities but I doubt there would be crazy combinations like drop troops + artillery.

c) Return, recruit and rearm: Send the regiment back to their homeworld to recruit more troops. If the regiment is unable to fight with their current strength and are worth the trouble, then shipping them back home to get fresh troops from their homeworld and keeping the experienced survivors in the regiment is probably the best option for the regiment. (As an alternative, rather than going back home and returning they might just send the new troops to the front line and add them to the regiment there.)

d) Recruit locally: Tanith 1st does this in the novels but there are other mentions of such regiments as well. In White Dwarf there was a history of a drop trooper regiment that always recruited on the world they were fighting on. The base of the regiment was from planet I-don't-remember but rather than sending reinforcements from their homeworld they would recruit locally.

Per the OW rules, you'd likely fold the original regimental members into a mixed regiment, if you wished to preserve those characters. However, from a fluffy standpoint, methinks 10 members surviving from a ~1000 strong regiment wouldn't be worth the trouble. 10 troopers are in no way battlefield effective. Per the Departmento Munitorum, a likely scenario is Regiment X = KIA. Future notes: Found new Regiment at earliest opportunity. There's be no collection of the flag, and as picky as the Departmento is, they aren't gonna come looking for ten lasrifles, 10 sets of flak armor and 10 pairs of combat boots. It's not worth the expenditure. Best case scenario, if on the victorious side? The ten poor buggers can find a way out of their last theatre of operations, hawk their gear and use the funds to eke their way as Imperial citizens. If they were on the losing side of a battle, the future looks...dim.

I say screw convention.

Here's a campaign model I wanted to run someday;

The players lost. Their forces are routed, they are among the desperate few survivors who are taken as prisoners of war or forced to survive. Cue evading enemy patrols and foraging for resources, and for all your burning hatred of the Munitorum and its constant errors, you really wish you had it right now.

Fight. Survive. Escape.

As always when the fluff doesn't depict the situation, look to history. If a unit is practically wiped out, it is not administratively merged with another unit. That happens only when both units still have usuable combat value and just need merging to bolster their sturdiness/ability to incur casualties. A destroyed unit is either retired from the books or it will be reformed somewhere else with fresh drafts.

The few survivors are usually ignored, depending on how the unit was destroyed and what happened to the rest of the army.

During WWII, there were many units who suffered such fates, especially on the British side during the desert campaigns and the retreat from Burma and on the German side during the desert campaign and on the eastern front.

Such survivors typically face three possible futures:

1) They are merged with other odds & sodds and form a temporary force (which may actually last a long time) to either bolster regular units or to serve as a regular unit. As such a force has poor morale, little internal cohesion and probably little experience working together, they tend to get hammered when encountering the enemy.

2) They are retained as a small unit and given rear-area duties. Armies tend to accumulate many of these broken units and can always use more men to do the sh*tty jobs in the rear and these troops usually have little combat value anyway after their ordeal.

3) They fall through the cracks. Their parent unit is disbanded and they are no longer on the books and thus have to survive amongst the rear-area units while scrounging for supplies. Or they can turn to banditry to survive. Or they get orders to rest and rebuild and are then promptly forgotten. This scenario is often depicted in movies (e.g. oddball's men in Kelly's Heroes).